Ron Jensen: fixed a potential NaN and Segfault in JSBSim propeller code

Ron Jensen: fixed a potential NaN and Segfault in JSBSim propeller code
The property /fdm/jsbsim/propulsion/engine/prop-induced-velocity_fps
gives wrong answers, and can become NaN under certain conditions. When thrust is
negative and forward velocity is small we can take the square root of a negative
number. This could occur, for example, when using reverse thrusters on landing.
The value comes out much too high when alpha is near 180, such as taxing with a
tail wind.